Good Afternoon, Rowghani,
Matt and Rick are correct. For a number of years, I used an 80mm Componon for 6 × 7. It covered, just barely. More recently, I have been using a newer 90mm Omegaron which covers 6 × 7 easily. The Omegaron is supposedly not of the quality of a Rodagon or a Componon, but I find that it gives superb results, at least up to 16 × 20. That lens might be a little difficult to find; I think I bought mine on E-Bay for an extremely reasonable price.
Konical